Various processes for the preparation of aminoacid chelating and metal ion sequestering agents are known including the coupling of an amine with chloracetic acid (U.S. Pat. No. 2,130,505) and the oxidation of ethanol amines (U.S. Pat. No. 2,384,816).
A more commonly used method utilizes hydrolysis of the appropriate nitrile, (U.S. Pat. No. 2,407,645) including variation such as separate formation, separation and hydrolysis of the nitrile (U.S. Pat. Nos. 2,164,781 and 2,205,995). In U.S. Pat. No. 2,855,428, ethylenediamine tetraacetonitrile (EDTN) is synthesized by introducing ethylenediamine amine to an acidic medium of formaldehyde and hydrocyanic acid. The nitrile is directly precipitated in the acidic medium as formed and recovered in good yields but requires a glass-lined (or other acid-resistant) reactor, good agitation and highly efficient heat transfer equipment since the reaction is highly exothermic and HCN will vaporize in the absence of high capacity cooling equipment. Reaction times are determined by the rate at which heat can be removed from the reaction medium. The process nevertheless has been practiced and continues to be practiced on a large scale ever since its initial commercial use.
U.S. Pat. No. 3,424,783 describes a variant wherein the cyanomethylation is conducted in acidic media. U.S. Pat. Nos. 3,644,444, 3,758,534, 3,679,729 and 3,714,223 disclose various specific acid pH ranges and temperature ranges, but in each case the tetracetonitrile is isolated and washed before saponificature as a prerequisite to pure EDTA acid preparation, requiring sophisticated equipment and extensive waste treatment operations. Failure to treat the reaction product as described therein results in lower yields.
U.S. Pat. Nos. 3,959,342 and 2,855,428 disclose formation of nitrilotriacetonitrile (NTN) and hydrolysis to the corresponding acid (NTA). Cyanomethylation (leading to NTN) is performed under acid conditions and provides yields of NTN in the range of 80 to about 95 wt % (based on ammonia or hexamethylenetetramine).
These older techniques involve close control of the exothermic reactions and moreover require "HCN rated" auxillary equipment. Not only is this more expensive, it precludes the use of more efficient heat exchange equipment. Moreover, the process can pose environmental hazards as a result of the HCN used and the by-products normally produced.